1. Field of the Invention
The present invention relates to a glass substrate for displays. In particular, it relates to a glass substrate for flat panel displays (the generic name for the flat display) such as plasma displays (PDP), TFT liquid crystal displays (TFT-LCD), STN liquid crystal displays (STN-LCD), plasma assist liquid crystal displays (PALC), electroluminescence displays (EL) and field emission displays (FED).
A flat panel display usually uses two glass substrates as the so-called front and rear panels in the case of a plasma display, or as the so-called array-sided and color filter-sided substrate in the case of a TFT liquid crystal display. The present invention relates to these glass substrates.
2. Discussion of Background
A flat panel display usually uses two glass substrates interposes a luminescent system or a light transmission control system between the two glass substrates. Typical glasses used for glass substrates include high strain point glass (such as a product of Asahi Glass Company named PD200) for plasma displays, non alkaline borosilicate glass (such as products of Asahi Glass Company named AN635, AN 100, etc. and a product of Corning Inc. named 1737) for TFT liquid crystal displays and soda lime glass (such as a product of Asahi Glass Company named AS) for STN liquid crystal displays.
These glass substrates are manufactured by the float method, the fusion method, the slit downdraw method and the like. Glass ribbon of even thickness made by these methods is cut into glass substrates of predetermined sizes.
Glass substrates are electric insulators around room temperature and charge up readily when brought into contact with or rubbed with other materials. Because in production of displays, microelectrodes are formed in various patterns on glass substrates, electrically charged glass substrates can cause dielectric breakdown of membrane electrodes. Besides, electrically charged glass substrates tend to attract dust and causes the problem called particle precipitation.
Therefore, a number of charge neutralizers such as ionizers are installed in production lines for flat panel displays. There are various types of ionizers such as the DC type, the AC type and the types utilizing vacuum ultraviolet rays or soft X rays, and the site of use determines the proper choice. However, these ionizers can not prevent glass substrates from electrification though they can neutralize electrically charged glass substrates.
As described above, charged glass substrates can cause problems of dielectric breakdown and particle precipitation. Electrification caused by contact with and separation from other parts during manufacture is an unavoidable phenomenon.
The above-mentioned contact/peeling electrification is considered to be explained by the following mechanism. When two approaching objects of different materials contact, charge carriers transfer from one object to the other across the interface between them. The distance between their surfaces is supposed to be about 0.2 to 0.8 nm. Electrons, ions, charged fragments from the surfaces of objects and the like are conceivable as charge carriers, but electrons are generally considered to predominantly transfer. As contacting two objects separate, part of the charges on them back off, and the rest remain as electrostatic charges (Yuji Murata: Kotai Butsuri, 27[7], 501-509(1992)).
During manufacture of liquid crystal display panels, glass substrates are brought into contact with and separated from metal plates which serve as holding plates for glass substrates and, in general, charge up negatively, i.e., electrons transfer from metal plates to glass substrates. The mobility of electrons is known to depend on the work function of the metal plate (Hiroyoshi Kitabayashi, et al.: Digest of the 1997 Spring annual meeting of the Japan Society of Applied Physics, 29a-NA-1,376 (1997)). Nickel imparts a smaller electrostatic potential to a glass substrate than aluminum when brought into contact with and separated from the glass substrate. Gold imparts a still smaller electrostatic potential to a glass substrate when brought into contact with and separated from it.
The magnitude of contact/peeling electrification of a glass substrate is known to depend on the contact surface area between the glass substrate and the metal. In other words, a glass substrate is assigned a smaller charge after contact with a metal plate having a rough surface, because the contact surface area between them is small. The charge on a glass substrate increases when contact with a metal is repeated, supposedly because the microscopic contact surface area increases (Hiroyoshi Kitabayashi, et al.: Digest of the 1996 annual meeting of the Institute of Electrostatics Japan, 31-32 (1996)).
The choice of the material and the surface roughness for the metal plate which comes into contact with a glass plate is crucial in order to prevent contact/peeling electrification of glass substrates. Gold is a perfect material for metal plates, as previously mentioned. However, formation of a gold coating on every facility in the production line that comes into contact with a glass substrate would cost a lot and is not a practical solution.
Another factor that affects the charge amount is relative humidity of the atmosphere, and it is known that electrification is unlikely to happen in high humidity. However, because of the problem that dew condensation is likely to arise in high humidity, in actual production processes, flat panel displays are manufactured in low humidity which favors electrification. Therefore, prevention of electrification in the manufacture of flat panel displays is an important subject.
The present invention has been accomplished to solve the above-mentioned problem and provides a glass substrate for a display with a thickness of from 0.3 mm to 6 mm, which has an average WCA (filtered center line waviness: JIS B0610) of from 0.03 to 0.5 xcexcm measured with a contact-type surface roughness measuring instrument using a phase compensation 2RC zone filter with a cutoff value of 0.8 to 25 mm over a measuring length of 200 mm.
Because of the dependence of the magnitude of contact/peeling electrification of a glass substrate on the contact surface area between the glass substrate and the metal, a glass substrate having the above-mentioned roughness has a small contact surface with a metal and gains a small charge.
In the present invention, the glass substrate for a display is preferably used for a plasma display panel, because most plasma display panels have large surface areas and can make use of the electrification preventing effect advantageously.
The present invention also provides a method of selecting a glass substrate for a display, which comprises inspecting the surface of a glass substrate for a display with a contact-type surface roughness measuring instrument using a phase compensation 2RC zone filter with a cutoff value of 0.8 to 25 mm over a measuring length of 200 mm and then selecting a glass substrate for a display with a thickness of from 0.3 to 6 mm having an average WCA (filtered center line waviness: JIS B0651) of from 0.03 to 0.5 xcexcm.
It is possible to obtain desirable glass substrates by screening glass substrates by this method even if glass substrates having intended properties are not produced in a 100% yield due to change in the production conditions. Namely, though there has been no method of identifying glass substrates which hardly take on electric charge, the present invention facilitates selection of glass substrates which hardly take on electric charge to be used.